He established his own design studio in 1962 as an industrial and interior designer. His Globe (or Ball) Chair was introduced to the public at the Cologne Furniture Fair in 1966 to immediate acclaim. Its critical success led Asko to establish a limited partnership with Aarnio in which he could design experimental furniture using new materials and modes of production.

Using plastics in a way that earlier Finnish designers like Tapiovaara had used wood, Aarino created ergonomic designs using state-of-the-art materials.

His organic forms, like the Globe Chair, represent an elegant Scandinavian approach to the use of synthetic materials demonstrating his belief that design should be beautiful and durable. 

Other innovative designs include the Bubble Chair which was similar to the Globe Chair but was clear and suspended from above; his floating Pastil Chair (similar to a solid inner tube), and the Formula1 Chair (modelled on a racing car seat); his Screw Table, as the name suggests, had the appearance of a flat head screw driven into the ground.

Aarnio's designs were an important aspect of 1960's popular culture, and could often be seen as part of sets in film (eg James Bond) and TV (eg The Prisoner). Instantly recognisable, the designs are ideal for such productions with their simple but striking geometric forms. They are as popular today as ever - the Globe Chair appears in the Marks & Spencer Christmas advertising campaign for 2006 and Richard Branson has suspended several Bubble Chairs in Virgin Atlantics First Class Lounge at Heathrow.

Aarnio is still working as a designer and in 2005 he joined the well established Finnish company Artek as one of their freelance designers.